Microchip wafers can be processed by flowing processing gases over the surface of a wafer. Many gas flow processing steps are required to accomplish the required etches and depositions, and uniform processing conditions must be maintained over the wafer surface for the processing to yield functional microchips. The processing equipment also must be carefully designed to limit the number of impurities that are introduced. High yields of good microchips are only achievable with careful process control and minimal impurities.
Chemical vapor deposition (CVD) is one example of such a gas flow process. A susceptor holds the wafer in a processing chamber. The processing gases are directed evenly over the surface of the wafer by forcing the gases to flow through one or more perforated plates, commonly called showerheads and typically oriented parallel to the wafer. Due to the flow restriction offered by the showerhead, the gas accelerates while passing through the showerhead and flows evenly over the wafer surface. The processing gases are continually exhausted from the chamber after they pass the wafer.
A primary factor reducing the yield of good microchips from such a reactor is the unintended deposition of particles. Processing gases can contain small numbers of particle contaminants. Particle contaminants can also be generated from the various parts of the reactor and suspended in the gas. As the gas accelerates while passing through the showerhead, the entrained particles are also accelerated. Particles can be accelerated through conventional showerheads to high enough velocities to be inertially deposited on the wafer. Even if this acceleration is insufficient to cause inertial deposition, particles can still be deposited on the wafer by forces such as gravity and thermophoretic effects. There is a need for improvements to the design of processing chambers to reduce the contamination by particles and therefore increase the yield of good microchips.